Causes

In most cases, it's not clear what causes colon cancer. Doctors know that colon cancer occurs when healthy cells in the colon become altered.

Healthy cells grow and divide in an orderly way to keep your body functioning normally. But when a cell is damaged and becomes cancerous, cells continue to divide — even when new cells aren't needed. These cancer cells can invade and destroy normal tissue nearby. And cancerous cells can travel to other parts of the body.

Precancerous growths in the colon

Colon cancer most often begins as clumps of precancerous cells (polyps) on the inside lining of the colon. Polyps can appear mushroom-shaped, or they can be flat or recessed into the wall of the colon. Removing polyps before they become cancerous can prevent colon cancer.

Inherited gene mutations that increase the risk of colon cancer

Inherited gene mutations that increase the risk of colon cancer can be passed through families, but these inherited genes are linked to only a small percentage of colon cancers. Inherited gene mutations don't make cancer inevitable, but they can increase an individual's risk of cancer significantly.

The most common forms of inherited colon cancer syndromes are:

Familial adenomatous polyposis (FAP). FAP is a rare disorder that causes you to develop thousands of polyps in the lining of your colon and rectum. People with untreated FAP have a greatly increased risk of developing colon cancer before age 40.

Hereditary nonpolyposis colorectal cancer (HNPCC). HNPCC, also called Lynch syndrome, increases the risk of colon cancer and other cancers. People with HNPCC tend to develop colon cancer before age 50.

FAP, HNPCC and other, rarer inherited colon cancer syndromes can be detected through genetic testing. If you're concerned about your family's history of colon cancer, talk to your doctor about whether your family history suggests you have a risk of these conditions.

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